gn: Add missing NDEBUG define in Release

gn: Add missing NDEBUG define in Release

This is required at least on Windows as it's used by system headers to
control things like assert, etc.

R=brettw@chromium.org
BUG=335824, 354261
Committed: https://crrev.com/4d2ee242cdc6eb2ce8119802b7c51f8393c945f0
Cr-Commit-Position: refs/heads/master@{#293788}

[brettw, 2014-08-14 22:54:05]

lgtm

[scottmg, 2014-08-14 22:56:35]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2014-08-14 23:00:39]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/scottmg@chromium.org/475153002/1

[commit-bot: I haz the power, 2014-08-15 05:04:05]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  android_chromium_gn_compile_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)
  android_dbg_triggered_tests on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tri...)

[commit-bot: I haz the power, 2014-08-15 05:16:06]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-08-15 05:16:07]

Try jobs failed on following builders:
  android_chromium_gn_compile_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)

[scottmg, 2014-09-08 20:09:20]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2014-09-08 20:11:11]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/scottmg@chromium.org/475153002/20001

[commit-bot: I haz the power, 2014-09-08 21:34:24]

Committed patchset #2 (id:20001) as 30126e08ea8b2c31496a3f472593de971a8d806a

[commit-bot: I haz the power, 2014-09-10 03:48:15]

Patchset 2 (id:??) landed as
https://crrev.com/4d2ee242cdc6eb2ce8119802b7c51f8393c945f0
Cr-Commit-Position: refs/heads/master@{#293788}